The compression ratio (CR) of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). Generally, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. Variable Compression Ratio (VCR) engines have been developed wherein the compression ratio of each cylinder can be varied between a higher and a lower setting to improve engine performance. For example, the higher compression ratio setting may be used during knock-free conditions to take advantage of the high thermal efficiency while the lower compression ratio setting may be used during knock prone conditions. In the VCR engines, a linkage or other mechanism (e.g., an eccentric) may be coupled to the piston of each cylinder to mechanically vary the compression ratio between the higher and lower settings.
One example of a VCR engine is shown by Caswell at U.S. Pat. No. 4,469,055. Therein, during engine running, the CR of the engine is adjusted based on engine operating conditions. For example, the CR may be optimized for engine fuel efficiency or engine performance, or both. The CR calibration, that is the CR commanded as a function of engine speed and load, may be calibrated based on a prototype engine.
However, the inventors herein have identified potential issues with such systems. As one example, the adjustment of the CR during engine operation requires the actual CR to be known accurately. However, each engine may have a slightly different compression ratio (CR) in each cylinder, due to manufacturing tolerances. In a VCR engine, each component of the VCR mechanism may have manufacturing tolerances leading to significant part-to-part variation, in addition to the normal variation on non-VCR engines. A VCR calibration based on the average CR (that is, the average of the CR across all engine cylinders) may result in extra spark retard usage on those cylinders which have a higher-than-average CR, leading to a much lower efficiency on those cylinders. Use of premium manufacturing methods and/or “select fit” parts can be used to control CR differences between cylinders, but such approaches add significant cost. Since VCR engines raise the compression ratio as much as possible, they tend to be knock-limited over a large portion of the engine operating map, and for a large fraction of a drive cycle. Without knowing the actual CR of each cylinder, and the cylinder-to-cylinder variations, it may be difficult to optimize the CR calibration, resulting in engine performance losses.
In one example, the above issues may be at least partly addressed by a method comprising: actuating a variable compression ratio mechanism of an engine to mechanically adjust a target compression ratio of the engine in accordance with an updated calibration, the updated calibration based on each of fuel flow and peak torque of each cylinder at each compression ratio setting of the mechanism. In this way, CR optimization of a VCR engine is improved.
As one example, the actual CR of each cylinder of a VCR engine may be quantified as a function of each VCR mechanism setting. For example, the CR of each engine cylinder may be quantified first while operating the VCR engine at a lower CR setting. Then, the CR of each engine cylinder may be quantified while operating the VCR engine at a higher CR setting. Then, the fuel flow and maximum IMEP of each cylinder may be quantified as a function of each VCR mechanism setting. Further, the parameters may be quantified as a function of the existing engine operating conditions, such as engine speed, engine torque, fuel octane, inlet air temperature, humidity, etc. The fuel flow and IMEP of all cylinders may then be summed to quantify the total engine fuel flow and total IMEP of the engine as a function of each VCR mechanism setting, at the current operating conditions. Thereafter, at each engine operating condition where driver demand is below a threshold, the engine controller may select the VCR mechanism setting which gives the minimum total engine fuel flow. At each operating condition where driver demand is above the threshold, the controller may select the VCR mechanism setting which gives the maximum total engine IMEP. The threshold may be a pre-determined value, or it may be adjusted as a function of current engine speed, fuel octane, ambient temperature, humidity, etc.
In this way, the efficiency of a VCR engine may be improved by detecting and compensating for cylinder-to-cylinder variations in compression ratio. The technical effect of learning fuel flow and IMEP of all cylinders as a function of each CR setting of the VCR engine is that CR variations of the actual engine may be learned, instead of relying on a prototype engine which may be significantly different from the given engine. Further, the VCR engine can be calibrated without relying on expensive manufacturing methods and/or components. By selecting a CR setting for the VCR engine that corresponds to the lowest total engine fuel flow when operator torque demand is low, fuel consumption and carbon dioxide (CO2) emissions can be minimized. By selecting a CR setting for the VCR engine that corresponds to the highest total torque when operator torque demand is high, engine performance can be maximized. Overall, engine performance and fuel efficiency of a VCR engine can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.